X99 on mATX is still limited to 4 DIMMs instead of 8, so not all of the advantages carry over. Overall though, mATX compatibility on a scaled down itx design is, quite like the mATX with ATX, rife with possibilities.
2x Single Slot GPUs (R9 Fury X / Nano) or a single pcb dual gpu (Fury X2) with an add-in card. WC could potentially still be an option, but would be dependent on the actual spacing between the mATX board and the intake fans. Based on the ITX sketch with a rad and attached small res/d5, an matx + slim rad + 25mm fans would fit - if not the 27.5mm Darkside Slim, then likely the 22mm Silverstone Slim. Whether or not such a loop is wise in terms of temps is another story.
Note that in the above mentioned 2x card single slot configuration, this opens the spot between cards - for those with an M.2, it will have some room to breathe, and for those with an actual 1x pci-e slot, it could potentially be used.
2x Single Slot GPUs (R9 Fury X / Nano) or a single pcb dual gpu (Fury X2) with an add-in card. WC could potentially still be an option, but would be dependent on the actual spacing between the mATX board and the intake fans. Based on the ITX sketch with a rad and attached small res/d5, an matx + slim rad + 25mm fans would fit - if not the 27.5mm Darkside Slim, then likely the 22mm Silverstone Slim. Whether or not such a loop is wise in terms of temps is another story.
Note that in the above mentioned 2x card single slot configuration, this opens the spot between cards - for those with an M.2, it will have some room to breathe, and for those with an actual 1x pci-e slot, it could potentially be used.